Corona detector with narrow-band optical filter

ABSTRACT

A corona detector for detecting a corona associated with a remote object. In one embodiment, the corona detector employs an optical filter having at least one passband centered at a wavelength corresponding with one of the molecular nitrogen emission spectrum second positive emission lines in the visible spectrum for filtering light from the remote object. A lens operatively coupled to the optical filter forms an image of the remote object, the lens having high transmissivity in the visible spectrum.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/890,920, filed Jul. 10, 1997, now U.S. Pat. No. 5,886,344,the entirety of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatuses for the detection ofelectrical corona discharge.

2. Description of the Related Art

High-voltage electrical apparatus often are surrounded by a corona whichis discharged by the apparatus when the surrounding air begins to loseinsulating qualities. For example, apparatus such as electrical powertransmission lines, transformer and substation insulators and bushings,high-voltage power supplies, and the like often have coronas associatedtherewith when the high electric field of the device causes thesurrounding air to begin to conduct rather than insulate. Thus, it isdesirable to detect the position and extent of such coronas in order todetect and address equipment failure. These coronas, which are alsosometimes referred to as corona discharges, will be referred to hereinsimply as coronas.

Such coronas are typically most easily visible or detectable by varioustechniques in darkness. However, there is a need to be able to detectcoronas even when there is not complete darkness, i.e. when there isambient light such as sunlight or artificial indoor light. One techniqueused to detect and identify the general position of coronas involves theuse of ultrasonic microphones. However, ultrasonic microphones do notprovide an image and thus cannot precisely locate the source of mostcorona discharges.

Conventional night-vision equipment which incorporates image intensifiertubes is also sometimes used to locate corona discharges, and can beused to provide an image of a corona, in addition to detecting thecorona, unlike ultrasonic techniques. Unfortunately, conventionalnight-vision equipment has relatively poor sensitivity to the opticalenergy emitted by a corona discharge, and much better sensitivity toboth sunlight and artifical lighting, requiring the equipment to beoperated in virtually complete darkness and giving poor sensitivity tocorona. This is inconvenient and expensive.

There is, therefore, a need for improved corona detection techniques.

SUMMARY

A corona detector for detecting a corona associated with a remoteobject. In one embodiment, the corona detector employs an optical filterhaving at least one passband centered at a wavelength corresponding withone of the molecular nitrogen emission spectrum second positive emissionlines in the visible spectrum for filtering light from the remoteobject. A lens operatively coupled to the optical filter forms an imageof the remote object, the lens having high transmissivity in the visiblespectrum.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become more fully apparent from the followingdescription, appended claims, and accompanying drawings in which:

FIG. 1 is a block diagram of a corona detector employing an opticalfilter to select one or more optical wavelengths associated withmolecular nitrogen second positive emission lines in the UV spectrum, inaccordance with an embodiment of the present invention; and

FIG. 2 is a block diagram of a corona detector employing an opticalfilter to select one or more optical wavelengths associated withmolecular nitrogen second positive emission lines in the visiblespectrum, in accordance with an alternative embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The light emitted by corona discharges in air is heavily concentrated ina relatively small number of very narrow bandwidths, typically a fewnanometers (nm) or less. The corona emission wavelengths are primarilyin the ultraviolet (UV) spectrum, with very weak emission intensitybetween the peaks of these bands. Much more than half of the totalintensity emitted by a corona discharge is emitted at wavelengthsshorter than 380 nm; the strongest emission is typically in a verynarrow band centered around 337.1 nm. Additional reasonably strongemission occurs at shorter wavelengths, down to at least 295 nm; otherstrong emission occurs at other wavelengths, as described below. Thisemission spectrum of a corona in ordinary air, and particularly itsstrong UV spectrum, is almost entirely due to the series of strongemission lines or bands of molecular nitrogen designated byspectroscopists as the second positive system or series. Therefore, mostof the optical energy emitted by a corona is in the form of UV light,and the corona emission spectrum is thus discontinuous and UV-rich.

In contrast with the discontinuous and UV-rich nature of the coronaemission spectrum, most sources of ambient light, including artificiallighting from lamps and sunlight, emit more or less continuous spectrathat cover hundreds of nm of bandwidth, and are more intense in thevisible and/or near-infrared portions of the spectrum than in the UVspectrum. The intensity of such ambient light drops rapidly at UVwavelengths (shorter than 400 nm).

Conventional night-vision equipment is mainly sensitive to visible andnear-infrared light, and thus has poor sensitivity to the UV lightgenerated by coronas. The present invention, in one embodiment, providesfor corona detection by employing an optical filter to select one ormore optical wavelengths associated with molecular nitrogen secondpositive emission lines in the UV spectrum, such that a corona dischargehas a relatively strong emission intensity at that wavelength. By usingsufficiently narrow bandwidth passbands of the optical filter for eachof the UV passband frequencies selected, the ambient or background lightis reduced relative to the UV light from the corona. Thus, in thepresent invention, an optical filter provides one or more sufficientlynarrow bandwidth passbands, each of which is centered around one of theUV molecular nitrogen second positive emission lines or around aclosely-spaced group of such lines. The present invention thereforeallows coronas to be imaged, and thus detected, since the ratio ofcorona emission light to ambient light is increased, as explained infurther detail below with respect to FIG. 1.

Referring now to FIG. 1, there is a block diagram of a corona detector100 employing an optical filter to select one or more opticalwavelengths associated with molecular nitrogen second positive emissionlines in the UV spectrum, in accordance with an embodiment of thepresent invention. Corona detector 100 comprises a narrow-band opticalfilter 101, a UV-transmitting lens 102, a UV image intensifier 103, animage inverter 104, and an eyepiece 105. As will be appreciated, thecomponent parts of detector 100, shown in FIG. 1, are preferablyassembled into a external housing (not shown) to form a single coronadetector unit. This unit may be held in the hands, or mounted on atripod or other support, and pointed toward the area of the apparatus tobe inspected. Typically, this area will be located 3 to 50 meters awayfrom the observer and the corona detector unit.

Optical filter 101 is preferably a bandpass, comb, or shortpass opticalfilter having one or more passbands in a selected part or parts of theUV portion of the spectrum and blocking light in the remaining portionsof the ultraviolet, visible, and near-infrared spectrum.

UV lens 102 is an image-forming lens capable of passing the wavelengthsselected by optical filter 101 with relatively small attenuation.Otherwise, if a UV-transmitting lens 102 were not employed, the UV lightpassed by optical filter 101 would be effectively filtered out by thelens. (Or, if the lens were in front of the optical filter, the lenswould filter the UV light out before reaching the optical filter.) Lens102 forms an image of a remote object for UV intensifier 103 in responseto input light received from the remote object. In one embodiment, UVlens 102 is composed of multiple lens elements, fabricated frommaterials with low attenuation in the 290-380 nm region, such as silica(quartz), calcium fluoride, magnesium fluoride, sapphire, and/orUV-transmitting optical glasses.

In one embodiment, as illustrated in FIG. 1, UV lens 102 comprises threelens elements in accordance with the classical Petzval portrait lensform, all elements are of fused silica, the relative aperture is f/4.5,the focal length is 100 mm at 337 nm, and the lens covers an image field18 mm in diameter with a modulation transfer function (MTF) greater than10% at 30 cycles per mm over at least half the image field. In anembodiment, UV lens 102 is not corrected for chromatic aberration, andmay be re-focused if the bandpass of optical filter 101 is changed tomatch different applications, for example in accordance with changes inambient light.

In one embodiment, UV lens 102 comprises a focusing mechanism. As willbe appreciated, other focal lengths, relative apertures, and other lensspecifications may be utilized in alternative embodiments, dependingupon the specific application. However, when varying thesespecifications, UV lens 102 preferably has low attenuation between 280and 380 nm and an MTF and angular field approximately matching that ofUV image intensifier 103.

The wavelengths selected by corona detector 100 are in the UV and aretherefore invisible to the human eye. Further, the intensity of thelight emitted by corona discharges is relatively low. Therefore, in oneembodiment, UV image intensifier 103 is employed to both amplify thefiltered light and convert it from UV to visible wavelengths. UV imageintensifier 103, in one embodiment, is an image-intensifier tubedesigned for good sensitivity (i.e. a photocathode responsivity ofroughly 10 mA/W or greater) at the UV wavelength(s) appropriate to theapplication, as discussed above with respect to selection of thebandpass of optical filter 101. In one embodiment, UV image intensifier103 comprises an input window composed of silica (quartz) or anotherUV-transmitting optical material, and comprises a photocathode of S-20,bialkalai, or similar UV-sensitive photocathode material. Thephotocathode diameter of UV image intensifier 103 may be 18 mm, 25 mm,or some other suitable size. The image intensifier power supply may beeither internal or external. In one embodiment, UV image intensifier 103is a "Gen II" proximity-focused micro-channel plate intensifier having aphoton gain of the order of 1,000-10,000. In an alternative embodiment,UV image intensifier 103 is a "Gen I" tube having lower gain andsensitivity but lower cost than Gen II type intensifiers. UV imageintensifier 103 further comprises output screen phosphors, such as P-20or P-43 (green emission) phosphors.

Image inverter 104 receives the output image from UV image intensifier103, and is used to present an image that is correctly oriented (top tobottom and right to left) when the user looks through eyepiece 105. Inone embodiment, image inverter 104 is included as a part of the housingof UV image intensifier 103. In an alternative embodiment, imageinverter 104 is not physically included within the image inverter 104housing, and comprises a fused fiber-optic bundle with a 180 degreetwist between input and output, an erecting prism assembly (e.g. Porropair, Pechan roof, etc.), or an inverting lens relay.

The MTF of image inverter 104 is, in one embodiment, approximately equalto or better than the MTF of UV image intensifier 103, to prevent imagedegradation, and the field coverage of image inverter 104 is sufficientto cover the UV image intensifier 103 phosphor screen diameter,typically 18 mm or 25 mm.

Eyepiece 105 is preferably of the same general type normally used foramateur astronomy, having a focal length of approximately 20-40 mm andcovering most or all of the field of the image inverter 104 output withan MTF roughly matching that of image inverter 104. All components ofcorona detector 100 are mounted in an enclosed housing (not shown) suchthat the correct mechanical spacings are maintained and the completeunit can be easily held in the hands and/or mounted on a rigid supportsuch as a camera tripod.

The center wavelength (maximum transmission wavelength) of opticalfilter 101 and the spectral width of its passband(s) are chosen so as topass an emission line, or group of lines, from the optical emissionspectrum of molecular nitrogen. Preferably, as explained above, thetransmitted line(s) are chosen from the optical wavelengths associatedwith molecular nitrogen second positive emission lines in the UVspectrum. The preferred passband(s) of optical filter 101 thuscorrespond with one or more relatively strong emission lines, forexample, one or more of the spectral lines having wavelength maxima at380.5 nm (also referred to as the 0,2 line of the molecular nitrogensecond positive emission spectrum), 375.5 nm (the 1,3 line), 371.0 nm(the 2,4 line), 357.7 nm (the 0,1 line), 353.7 nm (the 1,2 line), 337.1nm (the 0,0 line), 315.9 nm (the 1,0 line), 313.6 nm (the 2,1 line),311.7 nm (the 3,2 line), 297.7 nm (the 2,0 line), 296.2 nm (the 3,1line), or 295.3 nm (the 4,2 line).

Which spectral lines are selected by optical filter 101 by havingpassbands encompassing these lines is determined in accordance with theparticular application, for example whether the object to be inspectedfor possible coronas is outdoors or indoors, in sunlight or atnighttime, etc. For many applications such as the inspection ofelectrical power high-voltage transmission lines and substations, it isdesirable to be able to locate the corona during ordinary daylight, soas to avoid the inconvenience and expense of working at night. Forindoor applications, such as the testing of high-voltage componentsduring manufacture, it is desirable to work with the lights on.Therefore, selection of a narrow optical band in the UV spectrum wherethe corona emission is strong will substantially increase the ratio ofcorona emission light to background light, allowing a high-contrastcorona image to be produced by a UV-transmitting lens.

In the present invention, one or more passbands may be selected with theoptical filter in accordance with the nature of the ambient light. Inthe case of sunlight, ambient light intensity drops monotonically overthe entire UV spectrum, and becomes virtually undetectable atwavelengths shorter than 295 nm due to the absorption of atmosphericozone. Therefore, selection of the corona emission lines near 295-298 nmfor the bandpass of the optical filter will give a larger ratio ofcorona emission to background in the presence of sunlight than selectionof other corona emission wavelengths, such as the emission peak at 337nm. For bright sunlight, selection of the corona emission lines near295-298 nm is preferred.

Since the maximum absolute intensity of the corona emission occurs at337 nm, selection of this wavelength will normally be preferred when thebackground illumination is either dim sunlight or artificial light.Selection of intermediate lines, such as those near 316 nm, will yieldintermediate results.

A bandwidth of approximately 10 nm for the bandpass(es) of opticalfilter 101 has been found to be useful in obtaining these advantages,but using a bandwidth much broader than 10 nm begins to reduce theseadvantages (although there may be no detectable difference between theperformance with a 10 nm bandwidth bandpass and a 9 or 11 nm bandwidthbandpass). However, making bandwidth(s) of the bandpasses of opticalfilter 101 narrower can improve the performance significantly for someapplications. It has been empirically determined that an optimum filterbandwidth is about 1.0 to 10 nm for indoor applications, and about 0.001to 0.01 nm for outdoor applications, although the practical lower limiton the bandwidth of commercially-available optical filters at thepresent time is about 0.1 to 1.0 nm. Therefore, an optical filterbandpass having a bandwidth of 10 nm or less is, in general, preferred.

Thus, in one embodiment, each passband of optical filter 101 has abandwidth of 0.1 to 10 nm, for selection of one line or a closely-spacedgroup of lines, where "bandwidth" is defined conventionally to mean thefull width of the optical passband measured from the half maximumtransmission points, sometimes referred to as full width at half maximumor FWHM. The rejection band of optical 101 filter is preferably chosento reject (with attenuation at least 20 dB relative to the transmissionpeak) other light wavelengths within the range of approximately 290 toapproximately 1,200 nm. Rejection at wavelengths shorter than 290 nm isnot needed because there is very little background (ambient) light atthese wavelengths, and rejection at wavelengths longer than 1,200 nm isnot needed because UV image intensifiers such as intensifier 103typically are insensitive to wavelengths longer than roughly 1,000 nm.

The strongest corona emission normally occurs at the 337.1 nm emissionline, but the largest ratio of corona emission to ambient backgroundlight normally occurs at emission wavelengths shorter than 300 nm.Therefore, the selection of an optimum filter depends on the amount andtype of background light for a particular application. For example,optical filter 101 having a passband from about 294-299 nm may be foruse in bright outdoor daylight, and a passband centered on 337 nm may beused in dim outdoor light or indoors under artificial light, sincesunlight is very rich in UV at wavelengths longer than about 300 nm,whereas artificial light is relatively weak in UV at wavelengths shorterthan about 350 nm. In one embodiment, therefore, optical filter 101 ismounted in a removable mount, such that the user can select differentfilters for different applications (e.g. indoors versus outdoors, brightversus dim ambient light).

In use, UV lens 102 of corona detector 100 is focused so that it formsan image of the electrical apparatus or other remote object to beinspected at the photocathode of UV image intensifier 103. Eyepiece 105receives the output of image inverter 104 and is preferably focused toprovide a focused image of the remote object and any corona, for exampleto a user's eye looking into eyepiece 105. Since a small amount of UVlight within the passband(s) of optical filter 101 will normally bepresent, a dim image of the remote object will be seen in the eyepiece.If a corona discharge is present, a bright image of this discharge willbe seen at the corresponding location on the object being inspected.Empirical testing of the invention confirms that corona dischargesinvisible to other methods can be detected with useful sensitivity atuseful ranges. If necessary, the user may select from a variety ofnarrow-band optical filter(s) to obtain the maximum contrast of coronaagainst background, or the maximum sensitivity for detecting weakcoronas in dim background light.

Thus, in using corona detector 100, by combining at least one narrowfilter centered on one of the strong lines of the molecular nitrogenemission spectrum (especially the 0,0 line of the second positivesystem), the corona image brightness is reduced only slightly comparedwith the use of a broadband UV filter, whereas the background(interfering) light from other sources is reduced by a much greateramount, thus greatly increasing the signal-to-background ratio andmaking it possible to view corona at much greater levels of backgroundillumination.

As will be appreciated, although the present embodiment disclosed hereincontains an image inverter 104 and eyepiece 105 to allow for humanviewing, in alternative preferred embodiments other detection methodsmay be employed. For example, in one alternative embodiment, anelectronic detector can be positioned to receive the image of UV imageintensifier 103 (or incorporated therein), and suitably configured to beable to automatically detect any corona present in the image.Charge-coupled device (CCD) arrays or other solid-state electronic imagedetectors such as complementary metal oxide semiconductor (CMOS) imagertechnology may also be employed instead of an eyepiece and inverter tocapture the image on the phosphor screen of UV image intensifier 103.The image so captured may then be displayed on a monitor for viewing byhuman user or processed by an image processor configured toautomatically detect coronas.

As will be appreciated, although corona detector 100 of FIG. 1 isillustrated with optical filter 101 situated in front of UV lens 102, sothat light from the remote object passes through optical filter 101before passing through UV lens 102, in alternative embodiments, UV lens102 may be situated in front of optical filter 101, or optical filter101 may be situated within UV lens 102.

Referring now to FIG. 2, there is shown a block diagram of a coronadetector 200 employing an optical filter to select one or more opticalwavelengths associated with molecular nitrogen second positive emissionlines in the visible spectrum, in accordance with an alternativeembodiment of the present invention. Corona detector 200 comprises anarrow-band optical filter 201, a lens 202, an image intensifier 203, animage inverter 204, and an eyepiece 205. As will be appreciated, thecomponent parts of detector 200 are preferably assembled into a externalhousing (not shown) to form a single corona detector unit, as describedabove with reference to detector 100 of FIG. 1.

As described above, corona detector 100 of FIG. 1 employs optical filter101 to select one or more optical wavelengths associated with molecularnitrogen second positive emission lines in the ultraviolet spectrum.Because most sources of ambient light emit more or less continuousspectra that cover hundreds of nm of bandwidth, and are more intense inthe visible and/or near-infrared portions of the spectrum than in the UVspectrum, corona detector 100 allows coronas to be imaged by using anoptical filter of sufficiently narrow bandwidth to reduce the ambientlight relative to the UV light from the corona.

Corona detector 200, however, uses emision lines in the visible portionof spectrum. Although corona detector 200 may have somewhat reducedperformance compared to that attainable using UV emission lines (e.g.,decreased corona detection sensitivity and selectivity), it can also beless costly to manufacture than a UV system. As will be appreciated, inorder to produce corona imaging and detection products that can bemanufactured and sold for the lowest possible cost, particularly in lowto moderate volumes, it is desirable to utilize the maximum possiblenumber of mass-produced (as opposed to custom-made) optical components.The vast majority of mass-produced optical materials and components(transparent glass and plastic materials, lenses, filters, electronicimage detectors, and the like) perform well primarily in the visibleportion of the spectrum, while comparatively few are suitable for use inthe UV spectrum. Therefore, although some advantages are attainable byusing the UV emission lines for corona detection, production of suchdevices can be expensive.

Corona detector 200, using the visible emission lines, can bemanufactured at a lower cost because more of its components operate inthe visible-light portion of the spectrum rather than the UV portion. Inparticular, lens 202, narrow-band optical filter 201, and any electronicdetector components need only be operable in the visible spectrum, andthus may be obtained at lower cost and with greater commercialavailability or quantity.

Optical filter 201 is preferably a bandpass, comb, or shortpass opticalfilter having one or more passbands in a selected part or parts of thevisible portion of the spectrum and blocking light in the remainingportions of the spectrum.

Lens 202 is an image-forming lens capable of passing the visiblewavelengths selected by optical filter 201 with relatively smallattenuation, i.e., lens 202 has high transmissivity in the visiblespectrum. Lens 202 forms an image of a remote object for imageintensifier 203 in response to input light received from the remoteobject. In one embodiment, lens 202 is composed of multiple lenselements, fabricated from materials with low attenuation in the visibleportion of the spectrum.

The wavelengths selected by corona detector 200 are visible to the humaneye, and need not be converted to visible wavelengths, unlike in coronadetector 100. Therefore, in one embodiment, image intensifier 203 isemployed to amplify the filtered light. Image intensifier 203, in oneembodiment, is an image-intensifier tube designed for good sensitivity(i.e. a photocathode responsivity of roughly 20 mA/W or greater) at thevisible wavelength(s) appropriate to the application, as discussed abovewith respect to selection of the bandpass of optical filter 201. Thephotocathode diameter of image intensifier 203 may be 18 mm, 25 mm, orsome other suitable size. The image intensifier power supply may beeither internal or external. In one embodiment, image intensifier 203 isa "Gen II" proximity-focused micro-channel plate intensifier having aphoton gain of the order of 1,000-20,000. In an alternative embodiment,image intensifier 203 is a "Gen I" tube having lower gain andsensitivity but lower cost than Gen II type intensifiers.

Image inverter 204 receives the output image from image intensifier 203,and is used to present an image that is correctly oriented (top tobottom and right to left) when the user looks through eyepiece 205. Inone embodiment, image inverter 204 is included as a part of the housingof image intensifier 203. In an alternative embodiment, image inverter204 is not physically included within the image inverter 204 housing,and comprises a fused fiber-optic bundle with a 180 degree twist betweeninput and output, an erecting prism assembly (e.g. Porro pair, Pechanroof, etc.), or an inverting lens relay.

The MTF of image inverter 204 is, in one embodiment, approximately equalto or better than the MTF of image intensifier 203, to prevent imagedegradation, and the field coverage of image inverter 204 is sufficientto cover the image intensifier 203 phosphor screen diameter, typically18 mm or 25 mm.

Eyepiece 205 is preferably of the same general type normally used foramateur astronomy, having a focal length of approximately 20-40 mm andcovering most or all of the field of the image inverter 204 output withan MTF roughly matching that of image inverter 204. All components ofcorona detector 200 are mounted in an enclosed housing (not shown) suchthat the correct mechanical spacings are maintained and the completeunit can be easily held in the hands and/or mounted on a rigid supportsuch as a camera tripod.

As described above, corona detector 200 utilizes the emission lines fromthe second positive series of neutral molecular nitrogen that fallwithin the visible portion of the spectrum, and that are of sufficientrelative intensity to be practically useful for corona imaging anddetection applications. In particular, the visible emission lines thatare preferably used for this purpose are at the following approximatewavelengths, expressed in nm; the spectroscopic designation of each linein terms of its quantum numbers is given in parenthesis following eachentry:

399.84˜400 (1,4)

405.94˜406 (0,3)

409.48˜409 (4,8)

414.18˜414 (3,7)

420.05˜420 (2,6)

426.97˜427 (1,5)

434.36 and/or 435.50˜435 (0,4) & (4,9)

Therefore, the center wavelength (maximum transmission wavelength) ofoptical filter 201 and the spectral width of its passband(s) are chosenso as to pass one or more of the above emission lines or groups ofemission lines, depending on the application. A bandwidth ofapproximately 10 nm for the bandpass(es) of optical filter 201 has beenfound to provide useful results, but using a bandwidth much broader than10 nm begins to reduce these advantages (although there may be nodetectable difference between the performance with a 10 nm bandwidthbandpass and a 9 or 11 nm bandwidth bandpass). However, makingbandwidth(s) of the bandpasses of optical filter 201 narrower canimprove the performance significantly for some applications. It has beenempirically determined that an optimum filter bandwidth is about 1.0 to10 nm for indoor applications, and about 0.001 to 0.01 nm for outdoorapplications, although the practical lower limit on the bandwidth ofcommercially-available optical filters at the present time is about 0.1to 1.0 nm. Therefore, an optical filter bandpass having a bandwidth of10 nm or less is, in general, preferred.

As will be appreciated, the above-listed wavelengths are to some extentvisible to the unaided human eye, so that the image intensifier 103,CCD, or other electronic image detector or converter equipment which areor may be employed with UV detector 100, are not necessarily required invisible detector 200.

Thus, although the use of such components, particularly the imageintensifier, will give greater sensitivity to weak corona sources,corona detector 200, even if it does not employ image intensifier 203,has adequate sensitivity for many applications and can be produced forlower cost. Therefore, in one embodiment of visible-spectrum coronadetector 200, image intensifier 203 is not employed. In alternativeembodiments, however, image intensifier 204 may also be employed forenhanced performance.

Which spectral lines are selected by optical filter 201 by havingpassbands encompassing these lines is determined in accordance with theparticular application, for example whether the object to be inspectedfor possible coronas is outdoors or indoors, in sunlight or atnighttime, and the like. Therefore, the selection of an optimum filterdepends on the amount and type of background light for a particularapplication.

For direct visual observation utilizing the human eye as the detectordevice, instead of electronic detection components, lines at 426.97 or434.36 nm (approximately 427 or 435 nm) are preferred for mostapplications. In this case, for example, optical filter 201 has a centerwavelength and passband bandwidth wide enough to pass both these lines,e.g. a center wavelength of approximately 431 nm and a 10 nm bandwidthpassband. For observation with image intensifier 203, a CCD, or otherelectronic detector, lines at 399.84, 405.94, 426.97, or 434.36 nm arepreferred for most applications. The lines at 409.48, 414.18, and 435.50nm are generally weak, but may be preferred for certain special cases.

In use, lens 202 of corona detector 200 is focused so that it forms animage of the electrical apparatus or other remote object to be inspectedat the photocathode of image intensifier 203 (or the image is provideddirectly to image inverter 204 by lens 202 in the case where imageintensifer 203 is not employed). Eyepiece 205 receives the output ofimage inverter 204 and is preferably focused to provide a focused imageof the remote object and any corona, for example to a user's eye lookinginto eyepiece 205. If a corona discharge is present, a sufficientlyvisible image of this discharge will be seen at the correspondinglocation on the object being inspected. Empirical testing of theinvention confirms that corona discharges invisible to other methods canbe detected with useful sensitivity at useful ranges. If necessary, theuser may select from a variety of narrow-band optical filter(s) toobtain the maximum contrast of corona against background, or the maximumsensitivity for detecting weak coronas in dim background light.

Thus, in using corona detector 200, by combining at least one narrowfilter centered on one of the strong lines of the visible molecularnitrogen emission spectrum (especially the 1,5 (426.97 nm) or 1,4(399.84 nm) lines of the second positive system), the corona is filteredto a lower degree than ambient light, thus providing a corona image.

As will be appreciated, although the present embodiment disclosed hereincontains an image inverter 204 and eyepiece 205 to allow for humanviewing, in alternative preferred embodiments other detection methodsmay be employed. For example, in one alternative embodiment, anelectronic detector can be positioned to receive the image of imageintensifier 203 (or incorporated therein), and suitably configured to beable to automatically detect any corona present in the image. CCD arraysor other solid-state electronic image detectors such as CMOS imagertechnology may also be employed instead of an eyepiece and inverter tocapture the image on the phosphor screen of image intensifier 203. Theimage so captured may then be displayed on a monitor for viewing byhuman user or processed by an image processor configured toautomatically detect coronas.

As will be appreciated, although corona detector 200 of FIG. 2 isillustrated with optical filter 201 situated in front of lens 202, sothat light from the remote object passes through optical filter 201before passing through lens 202, in alternative embodiments, lens 202may be situated in front of optical filter 201, or optical filter 201may be situated within lens 202.

It will be understood that various changes in the details, materials,and arrangements of the parts which have been described and illustratedabove in order to explain the nature of this invention may be made bythose skilled in the art without departing from the principle and scopeof the invention as recited in the following claims.

What is claimed is:
 1. An apparatus for detecting a corona associatedwith a remote object, comprising:(a) an optical filter having at leastone passband centered at a wavelength corresponding with one of themolecular nitrogen emission spectrum second positive emission lines inthe visible spectrum for filtering light from the remote object; and (b)a lens operatively coupled to the optical filter and for forming animage of the remote object, the lens having high transmissivity in thevisible spectrum.
 2. The apparatus of claim 1, wherein the opticalfilter is positioned in front of the lens so that the lens receiveslight filtered by the optical filter.
 3. The apparatus of claim 1,wherein the passband of the optical filter has a bandwidth of 10nanometers (nm) or less.
 4. The apparatus of claim 1, wherein:thepassband of the optical filter has a bandwidth of 10 nm or less; and thepassband of the optical filter is centered at a wavelength so that theoptical filter passes light at wavelengths of approximately 427 nm,approximately 435 nm, or approximately 427 nm and approximately 435 nm.5. The apparatus of claim 1, wherein:the passband of the optical filterhas a bandwidth of 10 nm or less; and the wavelength is approximately400 nm.
 6. The apparatus of claim 1, wherein:the passband of the opticalfilter has a bandwidth of 10 nm or less; and the wavelength isapproximately 406 nm.
 7. The apparatus of claim 1, wherein:the passbandof the optical filter has a bandwidth of 10 nm or less; and thewavelength is approximately 409 nm.
 8. The apparatus of claim 1,wherein:the passband of the optical filter has a bandwidth of 10 nm orless; and the wavelength is approximately 414 nm.
 9. The apparatus ofclaim 1, wherein:the passband of the optical filter has a bandwidth of10 nm or less; and the wavelength is approximately 420 nm.
 10. Theapparatus of claim 1, wherein:the passband of the optical filter has abandwidth of 10 nm or less; and the wavelength is approximately 427 nm.11. The apparatus of claim 1, wherein:the passband of the optical filterhas a bandwidth of 10 nm or less; and the wavelength is approximately435 nm.
 12. The apparatus of claim 1, wherein:the passband of theoptical filter has a bandwidth of 10 nm or less; and the passband of theoptical filter is centered at a wavelength so that the optical filterpasses light at approximately 427 nm, approximately 435 nm, orapproximately 427 nm and approximately 435 nm, the apparatus furthercomprising an eyepiece for direct viewing by the human eye of the imageprovided by the lens.
 13. The apparatus of claim 1, furthercomprising:(c) an image intensifier tube for amplifying the imagereceived from the lens and filtered by the optical filter to provide anamplified image.
 14. The apparatus of claim 1, further comprising:(d) animage inverter for inverting the amplified image provided by the imageintensifier tube to provide an inverted image; and (e) an eyepiece forviewing the inverted image provided by the image inverter.